Microporous polyethylene membranes are mainly used as battery separators, particularly as separators for lithium batteries such as primary and secondary lithium ion batteries. Such batteries can be used to power electrical and electronic equipment such as mobile phones, notebook-type personal computers, etc. Recently, investigations have been conducted relating to the use of lithium secondary batteries as a power source for electric vehicles and hybrid cars. Batteries for hybrid electric vehicles (HEVs) such as hybrid cars need a relatively high power output which generally leads to the selection of a relatively thick battery separator compared to batteries designed for lower power output. High separator porosity is also generally beneficial. Thick separators can be beneficial for improving the safety performance of high-power batteries, by, e.g., reducing the risk of catastrophic battery failure resulting from internal short circuits. However, the use of thick separators can lead to a relatively high internal electrical resistance of the battery, resulting in difficulty in obtaining high power output. Thick separators for batteries of relatively low internal resistance can be produced by increasing the separator's porosity, but such microporous membranes are disadvantageous in that they can exhibit low pin puncture strength which can lead to manufacturing difficulties. Although separators of relatively high porosity are generally desirable for use in HEV batteries, increasing separator porosity generally reduces the separator's heat shrinkage resistance, which can result in separators that are more easily broken when wound up, leading to short-circuiting between the battery's electrodes. The separators described in the following references are representative of conventional separator technology.
JP11-21361A discloses a porous polyethylene film having conventional surface strength and air permeability. The film is made using a high-molecular-weight polyethylene resin having a viscosity-average molecular weight of 300,000 or more and less than 1,000,000, which has a thickness ranging from 5 to 50 μm, an air permeability of 100 seconds/100 cc or more and less than 250 seconds/100 cc, a porosity ranging from 40 to 60%, a pin puncture strength of 350 gf/25 μm or more, a pin puncture elongation of 2.0 mm or more, and a heat shrinkage ratio of 10% or less (105° C., width direction). This porous film is produced by extruding a melt blend of the above high-molecular-weight polyethylene resin and a plasticizer in the form of a film, stretching the resultant extrudate at a temperature ranging from 40 to 110° C. after cooling, removing the plasticizer, heat-treating the resultant membrane at a temperature of 110-125° C., and then re-stretching the membrane from 1.5 to 2.5 fold.
JP11-21362A discloses a porous polyethylene film having a relatively high surface strength and pin puncture elongation. The film is made from a high-molecular-weight polyethylene resin having a viscosity-average molecular weight of above 300,000 but less than 1,000,000, a thickness ranging from 5 to 50 μm, an air permeability of 250-1,000 seconds/100 cc, a porosity ranging from 30 to 50%, a pin puncture strength of 400 gf/25 μm or more, a pin puncture elongation of 2.0 mm or more and a heat shrinkage ratio of 5% or less (105° C., width direction). This porous film is produced by extruding a melt blend of the above high-molecular-weight polyethylene resin and a plasticizer in the form of a film, stretching the resultant extrudate at a temperature ranging from 40° C. to 110° C. after cooling, removing the plasticizer, heat-treating the resultant membrane at a temperature ranging from 110° C. to 125° C., and re-stretching it in a range from 0.9 to 1.5 fold.
Japanese Patent 2657430A discloses a microporous polyolefin membrane having improved pore diameter and pore diameter distribution. The film is made from a polyolefin containing 1% by weight or more of a component having a molecular weight of 7×105 or more, and having a molecular weight distribution (weight-average molecular weight/number-average molecular weight) of 10-300, a porosity ranging from 35 to 95%, an average penetrating pore diameter ranging from 0.05 to 0.2 μm, a rupture strength of 0.2 kg or more at a 15-mm width, and a pore diameter distribution (maximum pore diameter/average penetrating pore diameter) of 1.5 or less. This microporous polyolefin membrane is produced by extruding a melt blend of the above polyolefin and a solvent through a die, cooling the resultant extrudate to form a gel-like composition, stretching it at a temperature ranging from the crystal dispersion temperature of the polyolefin to the melting point+10° C., removing the remaining solvent, re-stretching the resultant membrane at a temperature of the melting point of the polyolefin−10° C. or lower, and heat-setting it at a temperature ranging from the crystal dispersion temperature to the melting point.
It would be desirable to further improve microporous polyolefin membranes, especially for use as HEV-battery separators. In particular, it would be desirable to improve the process steps used to form microporous membranes for HEV-battery service, especially respecting the kinetic melt viscoelasticity of the polyolefins selected for the membranes, the stretching temperatures of the extruded gel-like sheets (before removing the membrane-forming solvents), and the stretching and heat treatment conditions of the microporous membranes after removing the membrane-forming solvents.
Accordingly, relatively thick microporous membranes are desired having improved permeability, pin puncture strength and heat shrinkage resistance characteristics, and, particularly, improved air permeability characteristics.